Abstract : One of the essential components of the interstellar medium is neutralatomic hydrogen. A classical, but efficient method to study thiscomponent is the observation of the hyperfine transition at 21 cmwavelength of neutral hydrogen. The observation of this emission linecan be supplemented usefully by the measurement of the line inabsorption on the continuum radiation of discrete radiosources. Theabsorption profile gives the optical depth of neutral hydrogen onthe line of sight, which, compared with the emission profile, makesit possible in theory to determine the column density and theexcitation temperature of hydrogen in the studied direction.

We present here observations of absorption profiles 21 cm in thedirection of 819 extragalactic radiosources, carried out with theNançay radio telescope (Chapter II). We discuss the efficiency ofthis instrument for this kind of measurement (Appendix to Chapter II).

Then we use the sample of detected absorption components (300 at |b| >10°) for a statistical analysis of nearby hydrogen clouds. The studyof radial velocities makes it possible to obtain the motion of the Sunrelative to local interstellar gas; in retrieving the average effectof differential galactic rotation, we détermine the average distanceof neutral hydrogen clouds from the galactic plane, as well as theradial velocity dispersion radial of these clouds (Chapter III).

We study some observational bias which affect our sample: the blendingof the spectral components coming from clouds with close radialvelocities, the presence of spurious spectral components due to thecontamination of the absorption profiles by the 21 cm line in emission(Chapter IV, 1 and 2). Then we present the histogram of the internalvelocity dispersion of the clouds (chapter IV, 5). We then try todetermine the distribution of optical depths of the clouds, as well asthe probability of meeting a cloud of given optical depth on a line ofsight, taking into account observational bias and sensitivity limits(Chapter IV, 4). The comparison between our results and thepredictions of recent descriptive models of the interstellarenvironment indicates that these models must be revised (Chapter IV,5).

To specify the relationship between dense molecular clouds and diffuseneutral hydrogen clouds, we carried out radio observations of the OHradical hydroxyl and the CO molecule in some of the directions whereabsorption to 21 cm is known. The presence of molecules in somediffuse clouds suggests a continuity between diffuse clouds andmolecular clouds (Chapter 7).